A technique that transmits and receives radio waves having different frequencies through a single antenna has been known as a trap load technique. In the trap load technique, for example, in a case of transmitting and receiving two radio waves having different frequencies such as a high frequency and a low frequency, an LC parallel resonant circuit (trap) that resonates at the high frequency is connected to a quarter of the wavelength of the high frequency so as to resonate the antenna at the high frequency. Because the electric current does not flow at the part where the trap is connected, the radio wave having the frequency corresponding to the quarter of the wavelength, that is, the radio wave having the high frequency is transmitted and received.
With regard to the radio wave having the low frequency, considering that the loaded trap serves as a reactance, the total length of the antenna is adjusted so that the antenna is resonated at the low frequency. As such, the radio wave having the low frequency is transmitted and received.
In this way, the radio waves having different frequencies can be transmitted and received by the single antenna. Such a multi-band antenna is, for example, described in JP11-55022A corresponding to U.S. Pat. No. 6,163,300.